Photonic crystals are optical structures including a periodic arrangement of a dielectric or metallo-dielectric material. The periodic arrangement exhibits a strong interaction with light, and affects the propagation of electromagnetic waves (EM) in a manner analogous to the way the periodic potential in a semiconductor crystal affects electron motion. The periodic arrangement defines allowed photon propagation bands and forbidden photon propagation bandgaps. Optical pumping is a process in which light is used to excite the medium or some of its constituents into other energy levels. Typically electrons are raised from a lower energy level in an atom or molecule to a higher one.